How are you doing a "full shut down"? I know that sounds like a silly question, but I've found some people define shutting down differently than others.
Do you mean you're just shutting down from Windows and it makes this noise or are flipping the switch on the back of the PSU or something?
What I call "full shutdown" is turning USB power
off in S5 (shutdown) state (RGB turns off as well). When I do a shutdown from Windows 10, the surge protector (I found the source of the sound), makes that arcing sound.
However, if USB power is turned
on is S5 state, the surge protector will not make any sound,
until I physically turn off the switch on the PSU or the socket.
Removing and changing the surge protector fixed the issue.
However, based on what I read online, the buzzing occurs when a dying surge protector experience a high-voltage spike.
Does that mean my PSU puts a high voltage spike when it fully turns off?
My PSU is a Thermaltake Toughpower Grand RGB 650W 80+ Gold Full Modular
Is this a bad PSU?